This invention relates generally to temperature monitoring in rapid thermal processing (RTP) of silicon wafers, and more particularly the invention relates to the welding of temperature-sensing thermocouples to silicon wafers for temperature monitoring in rapid thermal processing.
The trend toward smaller device geometries for VLSI circuits has created interest in short-time (one to 100 seconds), high-temperature processing of silicon. Examples include rapid thermal annealing (RTA) and rapid thermal processing (RTP) of ion-implanted layers, as disclosed in U.S. Pat. No. 4,331,485 for Method of Heat-Treating Semiconductor Material Using High-Intensity CW LAMPS. Another example is limited reaction processing (LRP) which is described in U.S. Pat. No. 4,544,418 for process for High-Temperature Surface reactions in Semiconductor Material. Such techniques use radiant heating (e.g. tungsten filament, quartz envelope lamps) to couple power into the silicon.
In order to take advantage of these techniques, it is necessary to measure and control the wafer-temperature-versus-time profile very accurately. Commercial RTP systems use either thermocouples attached to the silicon or pyrometers which are calibrated against thermocouples to monitor the semiconductor processing temperature.
A persistent problem is the method used to bond thermocouples to the silicon wafers which serve as temperature sensors. Poor thermal contact and slow response times can limit accurate measurement of the wafer transient and, in some cases, steady-state temperature. Conventionally, holes are drilled into the silicon wafer for insertion of the thermocouple. The thermocouple is then sealed in the hole with a ceramic cement. This process can introduce impurities into the environment which can adversely affect both the temperature sensor and devices fabricated in the silicon wafer. Further, the desired thermal contact is not easily reproduced using cement holes. Additionally, the integrity of the thermocouple can be jeopardized.
Applicants have heretofore suggested the use of thermocouples welded directly to the silicon wafer, but the technique for welding the thermocouples has not been disclosed. See, for example, Gibbons et al. "Limited Reaction Processing: Silicon Epitaxy," Applied Physics Letters, 47(7) Oct. 1, 1985.